Carbohydrate-binding domain of the POMGnT1 stem region modulates O -mannosylation sites of α-dystroglycan

Abstract

The dystrophin glycoprotein complex, which connects the cell membrane to the basement membrane, is essential for a variety of biological events, including maintenance of muscle integrity. An O-mannose–type GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man structure of α-dystroglycan (α-DG), a subunit of the complex that is anchored to the cell membrane, interacts directly with laminin in the basement membrane. Reduced glycosylation of α-DG is linked to some types of inherited muscular dystrophy; consistent with this relationship, many disease-related mutations have been detected in genes involved in O-mannosyl glycan synthesis. Defects in protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGnT1), a glycosyltransferase that participates in the formation of GlcNAc-β1,2-Man glycan, are causally related to muscle-eye-brain disease (MEB), a congenital muscular dystrophy, although the role of POMGnT1 in postphosphoryl modification of GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan remains elusive. Our crystal structures of POMGnT1 agreed with our previous results showing that the catalytic domain recognizes substrate O-mannosylated proteins via hydrophobic interactions with little sequence specificity. Unexpectedly, we found that the stem domain recognizes the β-linked GlcNAc of O-mannosyl glycan, an enzymatic product of POMGnT1. This interaction may recruit POMGnT1 to a specific site of α-DG to promote GlcNAc-β1,2-Man clustering and also may recruit other enzymes that interact with POMGnT1, e.g., fukutin, which is required for further modification of the GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan. On the basis of our findings, we propose a mechanism for the deficiency in postphosphoryl modification of the glycan observed in POMGnT1-KO mice and MEB patients.

3D Structure and Function of Glycosyltransferases Involved in N-glycan Maturation

“…The overall fold of human GnT-II consists of an eight-stranded twisted β-sheet with 12 α-helical segments and forms GT-A fold such as GnT-I ( Figure 3a). Among many glycosyltransferases with GT-A folds, the overall structure of GnT-II is similar to those of GnT-I and protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGNT-1) ( Figure 3b) [49]. POMGNT-1 transfers GlcNAc to the Man-O-Ser/Thr acceptor via the β1-2 linkage to form core M1 (GlcNAcβ1-2Man-O-Ser/Thr) or M2 (GlcNAcβ1-2(GlcNAcβ1-6)-Man) glycans.…”

Bacteriophage T4 long tail fiber domains

“…An analysis using HHpred suggests the notion that the protein may consist of two carbohydrate-binding domains. Residues 19-178 show homology to PDB entry 5GGN, the N-terminal domain of human protein O-mannose β-1,2-N-acetylglucosaminyltransferase (Kuwabara et al 2016), and residues 228-344 show homology to PDB entry 4XUP, the N-terminal CBM22-1-CBM22-2 tandem domain from the Paenibacillus barcinonensis XYN10CM protein (Sainz-Polo et al 2015). The two carbohydrate-binding domains may have evolved to bind the gp34 and gp36 proteins instead of carbohydrates, although it is not known which domain of gp35 binds which protein.…”

Muscular Dystrophy with Ribitol-Phosphate Deficiency: A Novel Post-Translational Mechanism in Dystroglycanopathy

“…POMTs and POMGnT1 were originally identified as causative genes for WWS and MEB, respectively [ 12–14 ]. CoreM1 itself is not directly involved in ligand binding but is thought to play a critical role in the synthesis of the ligand binding moiety that is built on other O -Man type glycans [ 22 ]. CoreM1 can be further modified with β1,6-GlcNAc branch (CoreM2) [ 20 ], but this structure accounts for a very minor population of glycans from skeletal muscle α-DG (<0.1%) [ 23 ].…”